Transient voltage compensation apparatus and power supply using the same

ABSTRACT

A transient voltage compensation apparatus and a power supply using the same are provided. The power supply mainly uses an energy transferring circuit coupled between an input terminal and an output terminal of a power converter. When a load of the power supply is switched in a very short time, a power coupled to the energy transferring circuit is retrieved to compensate the output of the power supply, such that the output voltage is kept steady, and the transient response of the power supply is increased.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of Taiwan applicationserial no. 95132146, filed Aug. 31, 2006. All disclosure of the Taiwanapplication is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to a switching power supply. Moreparticularly, the present invention relates to an apparatus used forcompensating the transient output voltage undershoot when the load of apower supply is switched from a light load to a heavy load, and forcompensating the transient output voltage overshoot when the load of thepower supply is switched from a heavy load to a light load, and a powersupply using the same.

2. Description of Related Art

As rapid development of various information and communicationequipments, the design of a switching power supply with high efficiencyhas become a kind of knowledge combined with engineering and experience.The switching power supply is used to switch the power in manyapplications, including computers, illumination converters, andtelecommunication equipments. Taking the DC to DC switching power supplyas an example, during the short time for the load to be switched fromthe light load to the heavy load, since the energy of the DC to DCswitching power supply cannot be supplied in time, the output voltage isreduced in a very short time, thus causing an undershoot. However, inmany applications, a voltage output must be steady, and the problem thatthe voltage output drops in a very short time must be limited within atolerable scope.

The USA manufacturer, Intel Corporation, has established a regulation ofVoltage Regulator Down (VRD) standard. In the VRD standard, the outputvoltage of the power supply is regulated following the demand of theCPU.

However, when the modern CPU is operated under a high load, its powerconsumption is up to hundreds of watts. When the CPU is operated under alow load, its power consumption is generally about 5 Watts. Generally,the core voltage of the modern CPU is approximately between 1V and 2V.Supposing the core voltage of a CPU is 1V, and supposing its powerconsumption is 100 Watts when being operated under a high load, at thistime, the power supply must supply a current of 100 A, and if the CPU isoperated under a low load, the power supply is only required to supply acurrent of, for example, 5 A.

If the CPU is switched from a low load to a high load, the load of thepower supply is also switched from a light load to a heavy load, and atthis time, the voltage of the power supply will be suddenly dropped.When the output voltage outputted from the power supply is excessivelydropped at instant, the CPU will be entered the protection state due tothe drop of the core voltage, and thereby the computer is crashed.Furthermore, the CPU may also be damaged. In addition, as for a computeruser, it is not acceptable that the computer is crashed when working.When the CPU is switched from operating under a high load into operatingunder a low load, the load of the power supply is also switched from theheavy load into the light load, and at this time, the voltage of thepower supply will be suddenly arisen, which not only causes anadditional power consumption of the CPU due to the increase of the corevoltage, but also may damage the CPU.

SUMMARY OF THE INVENTION

The present invention is directed to providing a transient voltagecompensation apparatus and a power supply using the same, wherein whenan output load of a power converter is switched from a light load to aheavy load, an additional compensation energy is supplied to the load,such that the output voltage is kept steady, and the transient responseof the power supply is increased.

The present invention is also directed to providing a transient voltagecompensation apparatus, which is used to control a power converter,wherein when an output load of the power converter is switched from aheavy load to a light load, an output voltage is kept steady, and thetransient response of the power supply is increased.

The present invention is further directed to providing a power supplyusing the transient voltage compensation apparatus, which is used toreduce the arising or dropping of an output voltage in a very short timewhen an output load is switched from a heavy load to a light load, suchthat the output voltage is kept steady, and the transient response ofthe power supply is increased.

A transient voltage compensation apparatus of the present invention isconnected to a power converter having a power output terminal. Thetransient voltage compensation apparatus comprises a first operationalamplifier, a DC blocking element, a second operational amplifier, and anenergy transferring circuit. The first operational amplifier is coupledto the power output terminal and has a compensation output terminal. TheDC blocking element has a first terminal coupled to the compensationoutput terminal and a second terminal. The second operational amplifieris coupled to the second terminal and a reference signal terminal andhas an energy control terminal. The energy transferring circuit iscoupled to the energy control terminal and has a power supply terminaland a compensation terminal, wherein the compensation terminal iscoupled to the power output terminal.

In the transient voltage compensation apparatus according to a preferredembodiment of the present invention, the energy transferring circuitcomprises an impedance element and a switching element. One terminal ofthe impedance element is the power supply terminal of the energytransferring circuit. A switching control terminal of the switchingelement is the control terminal of the energy transferring circuit, afirst switching terminal of the switching element is coupled to theother terminal of the impedance element, and a second switching terminalof the switching element is the compensation terminal of the energytransferring circuit.

The present invention provides a power supply, which comprises a powerconverter, a first operational amplifier, a switching comparatorcircuit, a DC blocking element, a second operational amplifier, and anenergy transferring circuit. The power converter comprises an inputterminal and an output terminal. The first operational amplifier iscoupled to the power output terminal and has a compensation outputterminal. The DC blocking element has a first terminal coupled to thecompensation output terminal and a second terminal. The switchingcomparator circuit is coupled to the compensation output terminal andreceives a saw-tooth signal, and generates a pulse width modulation(PWM) signal according to the difference between the signal of thecompensation output terminal and the saw-tooth signal, so as to controlthe power converter. The second operational amplifier is coupled to thesecond terminal and the reference signal terminal and has an energycontrol terminal. The energy transferring circuit is coupled to theenergy control terminal and has a power supply terminal and acompensation terminal, wherein the compensation terminal is coupled tothe power output terminal.

As for the power supply according to a preferred embodiment of thepresent invention, the energy transferring circuit comprises animpedance element and a switching element. One terminal of the impedanceelement is the power supply terminal of the energy transferring circuit.A switching control terminal of the switching element is the controlterminal of the energy transferring circuit, a first switching terminalof the switching element is coupled to the other terminal of theimpedance element, and a second switching terminal of the switchingelement is the compensation terminal of the energy transferring circuit.

On one aspect, the present invention uses an energy transferring circuitcoupled between an input terminal and an output terminal of the powerconverter. When the load of the power supply is switched from a lightload to a heavy load, an AC component of the voltage output by the poweroutput terminal is retrieved, and the on/off state of the circuitbetween both terminals of the energy transferring circuit is controlled,therefore, the output voltage does not drop in a very short time whenthe load is switched from the light load to the heavy load. On the otheraspect, the present invention uses an energy transferring circuitcoupled between the power output terminal and the ground. When the loadof the power supply is switched from the heavy load to the light load,an AC component of the signal output by the power output terminal isretrieved, and the on/off state of the energy transferring circuit iscontrolled. Then, the circuit between both terminals of the energytransferring circuit consumes the additional energy generated by thepower supply, such that the output voltage is steady, and the transientresponse of the power supply is increased.

In order to make the aforementioned and other objects, features andadvantages of the present invention comprehensible, preferredembodiments accompanied with figures are described in detail below.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary, and are intended toprovide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention.

FIG. 1 is a circuit diagram of a power supply using a transient voltagecompensation apparatus according to an embodiment of the presentinvention.

FIG. 2 is a circuit diagram of the transient voltage compensationapparatus according to the embodiment of FIG. 1.

FIG. 3 is an operation waveform diagram of the circuit according to theembodiment of FIG. 2 of the present invention.

FIG. 4 is an operation waveform diagram according to an embodiment ofFIG. 2 of the present invention.

FIG. 5 is a circuit diagram of a power supply using a transient voltagecompensation apparatus according to an embodiment of the presentinvention.

FIG. 6 is an operation waveform diagram of the circuit according to theembodiment of FIG. 5 of the present invention.

FIG. 7 is a circuit diagram of a power supply using a transient voltagecompensation apparatus according to an embodiment of the presentinvention.

FIG. 8 is an operation waveform diagram of the circuit according toembodiments of FIG. 5 and FIG. 7 of the present invention.

DESCRIPTION OF EMBODIMENTS

In the prior art of the previous power supply, the output voltage of thepower supply will be suddenly changed when the load is switched from theheavy load to the light load or light load to heavy load, which not onlycauses inconvenience to the user, but also damages load. Therefore, thepresent invention provides an apparatus for compensating the transientvoltage and a power supply using the apparatus. The embodiments of thepresent invention will be described below with reference to theaccompanying drawings.

FIG. 1 is a circuit diagram of a power supply using the transientvoltage compensation apparatus according to an embodiment of the presentinvention. Referring to FIG. 1, the power supply includes a powerconverter 10, a first operational amplifier 11, a DC blocking element12, a second operational amplifier 13, and an energy transferringcircuit 14. The first operational amplifier 11 is coupled to a poweroutput terminal V10 of the power converter 10 and has a compensationoutput terminal A11. The DC blocking element 12 has a first terminal anda second terminal, and the first terminal is coupled to the compensationoutput terminal A11. The second operational amplifier 13 is coupled tothe second terminal of the DC blocking element 12 and a reference signalterminal R12 and has an energy control terminal EC12. The energytransferring circuit 14 is coupled to the energy control terminal EC12and has a power supply terminal VP14 and a compensation terminal CS14,wherein the compensation terminal CS14 is coupled to the power outputterminal V10 of the power converter 10.

It should be noted that, although a possible configuration of thecircuits of the transient voltage compensation apparatus and the powersupply using the same have been described in the above embodiment,persons of ordinary skill in the art should know that the coupling anddesign manners of the energy transferring circuit 14 are different forvarious manufacturers, and thus the application of the present inventionis not limited to such possible configuration. In other words, as longas the energy transferring circuit 14 is coupled between the outputterminal of the power converter 10 and a compensation power source, andwhen the load is switched in a very short time, the power coupled to theenergy transferring circuit is retrieved to compensate the output of thepower supply, which has already met the spirits of the presentinvention.

Several embodiments are cited below, so as to facilitate those skilledin the art to easily implement the present invention.

FIG. 2 is a circuit diagram of the transient voltage compensationapparatus according to the embodiment of FIG. 1. Referring to FIG. 2,the circuit enclosed by the dashed line 20 is the circuit of thetransient voltage compensation apparatus according to an embodiment ofthe present invention, and the circuit enclosed by the dashed line 21 isthe conventional power converter and the control circuit thereof. Thecircuit in this embodiment includes a power converter 210, a firstimpedance circuit Z1, a second impedance circuit Z2, an operationalamplifier 211, a switching comparator circuit 212, a DC blocking element201, an operational amplifier 202, and an energy transferring circuit203. In this embodiment, the energy transferring circuit 203 includes atransistor M203 and a resistor R203 used as switching elements. Inaddition, the DC blocking element 201 is implemented as a capacitor. Itis assumed herein that the input voltage V_(in) is higher than theoutput voltage V_(o).

In the above circuit of FIG. 2, the power converter and the controlcircuit thereof 21 include a power converter 210, an operationalamplifier 211 and a switching comparator circuit 212. The operationalamplifier 211 receives an output voltage V_(o) (power output terminal)of the power converter and a first reference signal V_(ref), and thenoutputs a compensation signal V_(comp) (compensation output terminal)according to the difference between the output voltage V_(o) and thefirst reference signal V_(ref). The switching comparator circuit 212receives the compensation signal V_(comp) and a saw-tooth signalV_(saw), and generates a pulse width modulation (PWM) signal D accordingto the difference between the compensation signal V_(comp) and thesaw-tooth signal V_(saw), so as to control the power converter 210(power input terminal). However, the operation of these components isclearly known by persons of ordinary skill in the art, which thus willnot be described in great detail. Similarly, the power converter may beimplemented as, for example, buck, buck-boost, or various topologicalstructures (forward or flyback) derived there-from, which thus will notbe described in great detail.

FIG. 3 is an operation waveform diagram of the circuit according to theembodiment of FIG. 2 of the present invention. Referring to both FIG. 2and FIG. 3, when the light load is switched to the heavy load, theoutput current I_(o) arises in a very short time, and as a result, theoutput voltage V_(o) drops in a very short time. The transient voltagecompensation apparatus according to an embodiment of the presentinvention shares an operational amplifier 211 (the first operationalamplifier) of the conventional power supply. The operational amplifier211 outputs a compensation signal V_(comp) according to an outputreference signal V_(o)′ and a first reference signal V_(ref), so as tocontrol the power converter 210. Next, the DC component of thecompensation signal V_(comp) is filtered via the capacitor 201 (DCblocking element), and the AC compensation signal V_(comp,ac) is outputto the operational amplifier 202 (the second operational amplifier).

Then, the operational amplifier 202 outputs a control signal V_(c) ofthe energy transferring circuit (energy control terminal) aftercomparing the AC compensation signal V_(comp,ac) with the secondreference signal V_(comp,ref). The transistor M203 is turned on after itreceives the control signal V_(c) of the energy transferring circuit atthe gate (switching control terminal), and the input energy isintroduced into an output terminal of the power converter 210, so as tocompensate the output voltage V_(o). The resistor R203 is used to limitcurrent, so as to prevent the output voltage from being excessively highcaused by excessive compensation. The compensated output voltage V_(o2)is obtained through the above manner, as such, the drop of the outputvoltage in a very short time when the light load is switched from theheavy load is compensated, and the transient response of the powersupply is increased.

It should be appreciated by those skilled in the art that, theoperational amplifier 202 and the switching comparator circuit 212 inthe above embodiment may be implemented as a comparator or anoperational amplifier. In addition, besides the N-type MOS transistorM203 in this embodiment, the switching element may be implemented asP-type MOS transistor, P or N-type JFET transistor, or P or N-type BJTtransistor, etc. If the N-type transistor is used as a switchingelement, a high side driving circuit is required for driving the N-typetransistor, which belongs to the conventional art, and thus will not bedescribed herein in great detail. In addition, the resistor R203 may bereplaced by JFET transistor operated in the linear region, MOSFETtransistor operated in the linear region or BJT transistor operated inthe linear region, and thus the present invention is not limited tothis.

FIG. 4 is another operation waveform diagram according to an embodimentof the present invention. The above waveform mainly changes the originalsecond reference signal V_(comp,ref) to a second reference signalV_(comp,ref2) in an AC form. Therefore, the switching element (M203) maybe turned on ahead of time, thereby effectively reducing the outputunder voltage effect when the load is switched from light load to heavyload.

FIG. 5 is a circuit diagram of a power supply using a transient voltagecompensation apparatus according to an embodiment of the presentinvention. The circuit enclosed by the dashed line 50 is the transientvoltage compensation apparatus according to an embodiment of the presentinvention, and the circuit enclosed by the dashed line 51 is theconventional power converter and the control circuit thereof. Referringto FIG. 5, the circuit in this embodiment includes a power converter510, a first impedance circuit Z1, a second impedance circuit Z2, anoperational amplifier 511, a switching comparator circuit 512, a DCblocking element 501, an operational amplifier 502, and an energytransferring circuit 503. In this embodiment, the energy transferringcircuit 503 includes a transistor M503, an inductor L503 and a diodeD503 used as switching elements. Furthermore, the DC blocking element501 is implemented as a capacitor.

In the circuit of FIG. 5, the power converter and the control circuitthereof 51 include a power converter 510, an operational amplifier 511and a switching comparator circuit 512. The operational amplifier 511receives an output voltage V_(o) (power output terminal) output by thepower converter and a first reference signal V_(ref), and then outputs acompensation signal V_(comp) according to the difference between theoutput voltage V_(o) and the first reference signal V_(ref)(compensation output terminal). The switching comparator circuit 512receives the compensation signal V_(comp) and the saw-tooth signalV_(saw), and generates a pulse width modulation(PWM) signal D accordingto the difference between the compensation signal V_(comp) and thesaw-tooth signal V_(saw), so as to control the power converter 510(power input terminal). However, the operation of these components isknown by persons of ordinary skill in the art, which thus will not bedescribed in great detail. Similarly, the power converter may beimplemented as, for example, buck, buck-boost, buck-boost, or varioustopological structures derived there-from, which thus will not bedescribed in great detail.

FIG. 6 is an operation waveform diagram of the circuit according to theembodiment of FIG. 5 of the present invention. Referring to both FIG. 5and FIG. 6, when the load is switched from heavy load to light load, theoutput current I_(o) drops in a very short time, such that the outputvoltage V_(o) arises in a very short time. The transient voltagecompensation apparatus according to an embodiment of the presentinvention shares the operational amplifier 511 (the first operationalamplifier) of the conventional power supply. The operational amplifier511 outputs a compensation signal V_(comp) according to the outputvoltage V_(o) and the first reference signal V_(ref), so as to controlthe power converter 510. Then, the DC component of the compensationsignal V_(comp) is filtered via the capacitor 501 (DC blocking element),and the AC compensation signal V_(comp,ac) is output to the operationalamplifier 502 (the second operational amplifier). The operationalamplifier 502 outputs a control signal V_(c) (energy control terminal)of the energy transferring circuit after comparing the AC compensationsignal V_(comp,ac) with the second reference signal V_(comp,ref). Thetransistor M503 is turned on after it receives the control signal V_(c)of the energy transferring circuit at the gate (switching controlterminal). Then, the additional energy is consumed by the inductor L503and the diode D503, and a compensated output voltage V_(o2) is obtained,such that the increase of the output voltage in a very short time whenthe heavy load is switched to the light load is compensated.

Persons of ordinary skill in the art should appreciate that theoperational amplifier 502 and the switching comparator circuit 512 inthe above embodiment may be implemented as a comparator or anoperational amplifier, and thus the present invention is not limitedherein.

FIG. 7 is a circuit diagram of a power supply using a transient voltagecompensation apparatus according to an embodiment of the presentinvention. This embodiment differs from the embodiment in FIG. 1 in thatthe energy transferring circuit 503 is changed to be implemented as aBJT transistor Q503 and a resistor R503. The operation of thisembodiment is similar to that of FIG. 1. Similarly, the additionalenergy is consumed by the resistor R503, so as to obtain a compensatedoutput voltage V_(o2). However, it is known to those skilled in the artthat, the switching element may be implemented as a JFET transistor, orMOSFET transistor, and furthermore, the resistor R503 may also bereplaced with the JFET transistor operated in the linear region, MOSFETtransistor operated in the linear region or BJT transistor operated inthe linear region, and thus the present invention is not limited herein.

FIG. 8 is another operation waveform diagram according to an embodimentof the present invention. The above waveform mainly changes the originalsecond reference signal V_(comp,ref) to a second reference signalV_(comp,ref2) in an AC form, such that the switching element (M503,Q503) may be turned on ahead of time, and the output over voltage effectwhen the load is switched from heavy load to light load is effectivelyreduced.

In view of the above, the present invention employs an energytransferring circuit coupled between an input terminal and an outputterminal of the power converter. When the load of the power supply isswitched from light load to heavy load, the AC component of the voltageoutput by the power output terminal is retrieved, and the on/off stateof a circuit between both terminals of the energy transferring circuitis controlled. Therefore, the output voltage does not drop in a veryshort time when the load is switched from light load to heavy load, suchthat the output voltage is kept steady, and the transient response ofthe power supply is increased.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the present inventioncover modifications and variations of this invention provided they fallwithin the scope of the following claims and their equivalents.

1. A transient voltage compensation apparatus, connected to a powerconverter which has a power output terminal, comprising: a firstoperational amplifier, coupled to the power output terminal and having acompensation output terminal; a DC blocking element, having a firstterminal and a second terminal, wherein the first terminal is coupled tothe compensation output terminal; a second operational amplifier,coupled to the second terminal and a reference signal terminal andhaving an energy control terminal; and an energy transferring circuit,coupled to the energy control terminal and having a power supplyterminal and a compensation terminal, wherein the compensation terminalis coupled to the power output terminal.
 2. The transient voltagecompensation apparatus as claimed in claim 1, wherein the energytransferring circuit receives a control signal output by the energycontrol terminal, the power supply terminal is coupled to an inputterminal of the power converter, the compensation terminal is coupled tothe power output terminal, and the on/off state of a circuit between thepower supply terminal and the compensation terminal is determined by thecontrol signal.
 3. The transient voltage compensation apparatus asclaimed in claim 2, wherein the energy transferring circuit comprises: aswitching element, comprising a switching control terminal, a firstswitching terminal and a second switching terminal, wherein theswitching control terminal is connected to the energy control terminal,and the second switching terminal is the compensation terminal.
 4. Thetransient voltage compensation apparatus as claimed in claim 3, whereinthe energy transferring circuit comprises: an impedance element, coupledbetween the power supply terminal and the first switching terminal. 5.The transient voltage compensation apparatus as claimed in claim 4,wherein the impedance element is a resistor.
 6. The transient voltagecompensation apparatus as claimed in claim 1, further comprising: aswitching comparator circuit, coupled to the compensation outputterminal, receiving a saw-tooth signal, and generating a pulse widthmodulation(PWM) signal according to a difference between a signal of thecompensation output terminal and the saw-tooth signal, so as to controlthe power converter.
 7. The transient voltage compensation apparatus asclaimed in claim 1, wherein the DC blocking element is a capacitor. 8.The transient voltage compensation apparatus as claimed in claim 1,wherein a negative input terminal of the first operational amplifier iscoupled to the power output terminal, a positive input terminal thereofreceives a reference signal, and an output terminal thereof is thecompensation output terminal.
 9. The transient voltage compensationapparatus as claimed in claim 8, further comprising: a first impedancecircuit, having one terminal coupled to the power output terminal, andanother terminal coupled to the negative input terminal of the firstoperational amplifier; and a second impedance circuit, having oneterminal coupled to the negative input terminal of the first operationalamplifier, and another terminal coupled to the output terminal of thefirst operational amplifier.
 10. The transient voltage compensationapparatus as claimed in claim 1, wherein the energy transferring circuitreceives a control signal, the power supply terminal is grounded, thecompensation terminal is coupled to the power output terminal, and theon/off state of a circuit between the power supply terminal and thecompensation terminal is determined by the control signal.
 11. Thetransient voltage compensation apparatus as claimed in claim 10, whereinthe energy transferring circuit comprises: a switching element,comprising a switching control terminal, a first switching terminal anda second switching terminal, wherein the switching control terminal isconnected to the energy control terminal, and the second switchingterminal is the compensation terminal.
 12. The transient voltagecompensation apparatus as claimed in claim 11, wherein the energytransferring circuit comprises: an inductor and a diode, connected inparallel and coupled between the first switching terminal and the poweroutput terminal, wherein a cathode of the diode is coupled to the poweroutput terminal, and an anode thereof is coupled to the first switchingterminal.
 13. A power supply, comprising: a power converter, comprisinga power input terminal and a power output terminal; a first operationalamplifier, coupled to the power output terminal and having acompensation output terminal; a DC blocking element, having a firstterminal and a second terminal, wherein the first terminal is coupled tothe compensation output terminal; a switching comparator circuit,coupled to the compensation output terminal, receiving a saw-toothsignal, and generating a PWM signal according to a difference between asignal of the compensation output terminal and the saw-tooth signal, soas to control the power converter; a second operational amplifier,coupled to the second terminal and a reference signal terminal andhaving an energy control terminal; and an energy transferring circuit,coupled to the energy control terminal and having a power supplyterminal and a compensation terminal, wherein the compensation terminalis coupled to the power output terminal.
 14. The power supply as claimedin claim 13, wherein the energy transferring circuit receives a controlsignal output by the energy control terminal, the power supply terminalis coupled to the input terminal of the power converter, thecompensation terminal is coupled to the power output terminal, and theon/off state of a circuit between the power supply terminal and thecompensation terminal is determined by the control signal.
 15. The powersupply as claimed in claim 14, wherein the energy transferring circuitcomprises: a switching element, comprising a switching control terminal,a first switching terminal and a second switching terminal, wherein theswitching control terminal is connected to the energy control terminal,and the second switching terminal is the compensation terminal.
 16. Thepower supply as claimed in claim 15, wherein the energy transferringcircuit comprises: an impedance element, coupled between the powersupply terminal and the first switching terminal.
 17. The power supplyas claimed in claim 16, wherein the impedance element is a resistor. 18.The power supply as claimed in claim 13, wherein the DC blocking elementis a capacitor.
 19. The power supply as claimed in claim 13, wherein theenergy transferring circuit receives a control signal, the power supplyterminal is grounded, the compensation terminal is coupled to the poweroutput terminal, and the on/off state of a circuit between the powersupply terminal and the compensation terminal is determined by thecontrol signal.
 20. The power supply as claimed in claim 13, wherein theenergy transferring circuit comprises: a switching element, comprising aswitching control terminal, a first switching terminal and a secondswitching terminal, wherein the switching control terminal is connectedto the energy control terminal, and the second switching terminal is thecompensation terminal.